The scheme — to be funded by an annual £25,000 charge on residents in Britain who do not pay tax on overseas earnings — is likely to prove popular with voters and is a bold move designed to grab the initiative on tax in a looming election campaign.

Because of the extraordinary rise in the value of homes, millions have found that their estates are now worth far in excess of the present threshold, making them liable for a tax that was originally meant as a levy on the super-rich.

In 1997 the Treasury raised £1.68 billion from the tax. In the last financial year this rose to £3.6 billion, with a projected rise next year to £4 billion.

While only 33,000 estates paid the tax last year, the Tories claim that about nine million family homes are worth more than the threshold.

Sources close to Mr Osborne said that this would not be the end of his tax plans. He will also look at corporation tax and income tax. However, Mr Osborne is delaying releasing these details until he knows whether there will be an autumn election.

If no November poll is called, it is understood that the new proposals will be disclosed by the end of the year.

"We are the low-tax party," he said. "We have a new dividing line between a Labour Prime Minister who has taxed a generation out of home ownership and a Conservative government that will abolish stamp duty for first-time buyers.

"The dividing line between a Labour Prime Minister who takes away the homes of those who have saved all their lives and a Conservative government that takes people's homes and savings out of inheritance tax."

Despite pledging to match Labour's spending commitments, it is understood that Mr Osborne will make clear soon that the commitment ends in 2010 and he has room to make future costed plans on tax.

The total cost of the tax changes will be £3.5 billion, which will be paid for by the £25,000 non-domicile levy.

The Tories claim there are 150,000 such residents, many of whom would be able to off-set the charge against their tax bill in their home country.

But Labour said last night that the Tory figures were wrong. They claimed that the real figure for non-domiciles was between 114,000 and 120,000.

Also, they pointed out, that a lot of those "tax exiles" were doctors and nurses and that they would be hit hard.

Richard Lambert, the director general of the CBI, said: "The taxation of non-domiciled individuals needs to be carefully considered in the context of the contribution that they bring to the country as a whole. While the sums mentioned by Mr Osborne may be small for these people, we need to be wary of driving jobs and opportunities away from the UK."

The move on inheritance tax is seen as a "tax cut" to counter claims that Mr Cameron's party has abandoned core principles.

Mr Osborne said: "When inheritance tax was first introduced it was designed to hit the very rich. But the very rich hire expensive advisers to make sure they don't pay it. Instead, thanks to Gordon Brown, this unfair tax falls increasingly on the aspirations of ordinary people.

"So now well over a third of homeowners in Britain have the threat of inheritance tax hanging over them. These are people who have worked all their lives.

"People who have saved money all their lives; people who have already paid taxes once on their income; people whose only crime in the eyes of the tax man is that instead of spending their savings on themselves, they want to pass something on to their families."

In a highly personal attack on Mr Brown, he said: "If you want an election, Prime Minister, then get on and call it: because your cynicism and your fear will lose every time to our hope and our optimism."

In addition he praised the non-domiciles and said he would not tax all their income "as Gordon Brown has persistently threatened to do."

Of the £25,000 annual levy, he said: "It is easy to administer, difficult to avoid and strikes the right balance between a fair tax system and a competitive economy."

The plan to take 200,000 out of stamp duty on their first homes, coupled with this latest move, is designed to show voters a clear dividing line with Labour.

Mr Osborne also accused Mr Brown of wasting billions on the NHS and other failed schemes.

Alistair Darling, the Chancellor, said: "Initial costings by the Treasury show that George Osborne’s proposal would raise a maximum of £650?million, leaving him at least £2·9?billion short. He cannot afford the promises he is making. He cannot afford to cut inheritance tax."